archive/zip: improve Writer.Create documentation on how to add directories

FileHeader.Name also reflects this fact.

Fixes #24018

Change-Id: Id0860a9b23c264ac4c6ddd65ba20e0f1f36e4865
Reviewed-on: https://go-review.googlesource.com/97057Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/archive/zip/struct.go

@@ -82,7 +82,8 @@ const (
 type FileHeader struct {
 	// Name is the name of the file.
 	// It must be a relative path, not start with a drive letter (e.g. C:),
-	// and must use forward slashes instead of back slashes.
+	// and must use forward slashes instead of back slashes. A trailing slash
+	// indicates that this file is a directory and should have no data.
 	Name string
 
 	// Comment is any arbitrary user-defined string shorter than 64KiB.

src/archive/zip/writer.go

@@ -209,7 +209,8 @@ func (w *Writer) Close() error {
 // The file contents will be compressed using the Deflate method.
 // The name must be a relative path: it must not start with a drive
 // letter (e.g. C:) or leading slash, and only forward slashes are
-// allowed.
+// allowed. To create a directory instead of a file, add a trailing
+// slash to the name.
 // The file's contents must be written to the io.Writer before the next
 // call to Create, CreateHeader, or Close.
 func (w *Writer) Create(name string) (io.Writer, error) {